Green roofs could prevent sewage overflow in New York City

New York City’s sewage systems are old and can’t handle large amounts of snowmelt or stormwater. When large quantities of water flow into the combined sewer overflow systems, which make up 60 to 70 percent of the city’s sewers, they tend to spill over into New York City’s rivers.

It doesn’t help that the city is about 72 percent impervious, so water goes straight into drainage systems. Since May 2013, more than 45 incidents of sewage discharge have been reported. This past winter, untreated sewage overflowed into the Hudson and East Rivers on multiple occasions, sometimes for as long as seven hours.

But not all is lost. Rooftop gardens could be the solution, and New York City is working hard to help New Yorkers get their green on.

Brooklyn Grange, the largest rooftop farm and garden business in the US, is part of this initiative. The Brooklyn Grange itself spans 108,000 square feet, with locations in Queens and Brooklyn. The farms absorb 60,000 and 100,000 gallons of stormwater every rainfall.

“Over the course of the year, we’re keeping millions of gallons of water out of the sewer system,” Gwen Schantz, one of the founders and chief operating officer of the Brooklyn Grange, told TreeHugger.

Photo cred: Manon Verchot || The Brooklyn Grange in the Brooklyn Navy Yard stores up to 100,000 gallons of water per rainfall AND has a stunning view of Manhattan/CC BY 2.0

Outside of their own farms, Brooklyn Grange offers consulting and installation services all over the city for “environmentally friendly ecological vegetated installations,” as Gwen puts it. They work carefully with architects to make sure that buildings can hold the weight of a roof garden. This is especially important since a gallon of water weighs 8.3 pounds. If the weight isn’t distributed carefully, everything could go crashing down.

Blue roofs are another effective way of preventing sewage overflow. With the use of a blocking system, such as a small weir, they trap water on the roof and gradually let it down the drain pipes, acting as a sort of urban wetland that temporarily stores rainfall runoff. As with green roofs, blue roofs need to be inspected by an architect to ensure that the building can hold up the weight of the water. But green roofs tend to be better because not only do they absorb water, they also help evaporate the water away, so they permanently keep water out of the sewage system.

“Every square foot of plants and soil can really make a difference,” says Gwen.

The Department of Environmental Protection has also been making huge efforts to curb sewage overflow. Every year, they pour millions of dollars into grants that allow New Yorkers to start their own gardens. These gardens could absorb more than 5.7 million gallons of water every year, not to mention the millions of gallons already being absorbed by the existing parks.

Native New Yorkers know not to swim on Coney Island after a rain because of the sewage polluted storm water runoff, but if more people put out plants and grow green roofs, that might change.